Dirt Car Setup Tips

1/15We should never be afraid to make changes to our setups in order to match the changing track conditions that are so common when racing on dirt. Today, the top teams are not only making changes, but have learned exactly which ones will make the car better as the track surface evolves.

Change, change, everywhere a change... , a message from social protests of the '60s, could have been written in reference to dirt track racing. Throughout an event, most dirt tracks change surface characteristics, some in a dramatic way. Granted, there are some so-called dirt tracks that are always oiled and maintained to the same grip. These are exceptions and won't be addressed here. They are more like asphalt anyway, and a team racing a dirt car on those tracks would do well to set up their cars more closely to an asphalt car setup.

The most difficult tracks are the ones in which the type of dirt or clay needs a lot of moisture in order for the track crew to be able to groom the track. A promoter will often plow up a track during the week preceding the event, water it throughout the morning of the event, and then roll it while still pouring water on it until practice time. From then on, unless it is watered again, the track will begin to dry out and go through several phases of moisture content.

2/15The Hobby Stock setups (A) and the typical Late Model setups (B). For wet and sloppy conditions, usually the first situation we encounter during practice and possibly qualifying...

What makes these various conditions so hard on our setups is that the g-forces are changing along with the declining availability of grip as the track begins to dry out and go slick. As the g-forces change, our setups must change too. The difference is in the grip factor and mostly deals with the level of moisture present in the first inch or two of the surface material. Here is a list of phases of grip that a dirt team might encounter on a typical race day, along with the associated estimated g-forces that the car might experience:

1. Wet, sloppy, and rough: 1.6 to 1.7 g's (peak g's but very difficult for the driver to maintain a good line)

2. Very moist but graded and compacted: 1.5 to 1.6 g's (the best the track will be all day and very fast through the turns).

3. Hardened and more packed as the surface begins to dry out: 1.3 to 1.5 g's (still good corner speed while losing a little grip off the corner).

3/15...we need to set up the car stiffer to withstand the high g-forces as the car gets thrown around in the ruts.

4. Black slick, a condition in which the track still has enough moisture to keep the material packed, but has hardened and is taking rubber: 1.1 to 1.3 g's (becoming difficult entering and turning in the middle of the turns).

5. Dry, a condition in which the top layer is now drying out and losing material in the form of sand: 0.9 to 1.1 g's (loss of grip is now getting substantial, and the car will not turn well or get bite off the corners. Driver touch is most important in maintaining momentum).

6. Dry slick, a condition in which the track has lost considerable moisture and is now very dry and slick with little grip: 0.7 to 0.9 g's (The cars are now very slow in getting into and through the turns, and traction under power is very low. Small gains in mechanical grip can provide substantial gains in lap times).

We can see how it is possible to experience a high 1.7 g's all the way down to a very low 0.7 g's in a single day. There is no way a team can run well in all of the conditions while maintaining the same setup. What we need to do is think about how these conditions, one by one, affect a car and then set it up accordingly. We will examine some physical rules that deal with chassis dynamics so we can arrange the setup to meet the conditions.

Let's use two of the most popular types of dirt car-a touring Late Model and a metric-style Hobby Stock car. For each type of car, we will set up for the six conditions listed above. The changes will be realistic and will not involve cutting or welding the chassis. Time is a factor and we want to make positive changes that can be accomplished in a short period of time.

This track condition means the tires are getting a lot of grip and the surface tends to have ruts that grab the car hard. To be able to control the car, we need much stiffer springs and a more balanced setup front to rear. The car's center of gravity (CG) needs to be lower when the grip is high. A low CG number for a dirt car is around 17-18 inches off the ground.

5/15...as well as the four-link setup (B) are set so that the birdcage ends at the top and bottom will move in opposite directions as the chassis compresses on the right side and rebounds on the left side. This keeps the axle tube in the same position front to rear to provide zero rear steer.

Hobby Stock (HS): We might run a stiffer right-front (RF) spring and a stiffer right-rear (RR) spring. A typical spring layout would be: left-front (LF) = 800; RF = 900 (or a 750 with a full spring rubber, if spring changes are not allowed); left-rear (LR) = 200; RR = 225 (or a 175 with two full spring rubbers). The rear ride height would be set as low as possible, as this would raise the rear moment center and lower the CG to reduce the rear roll angle versus the front roll angle to help balance the setup. Any moveable weight would be mounted low and left in the car. If you have a choice, run tires with large grooves such as the all-terrain types for mud and snow. Run a higher pressure in all of the tires.

Late Model (LM): A similar situation would apply to the LM car. For now, use a stiffer (RR) spring with a high Panhard bar mounted on the right side for consistency. Keep the weight low and to the left, and use tires with wide grooving to release the wet mud easily. Keep the air pressure high.

6/15The Late Model setup is very similar to an asphalt setup when the dirt has a lot of traction. The car can be driven straight ahead, deep into the corners, and hard off the corner. The right-side chassis Panhard mount technique makes the car more stable for the high g-force conditions. We will move the bar over to the left later on.

A typical setup might be: LF = 400; RF = 450; LR = 225 (400 pounds swing arm); RR = 275 (500 pounds swing arm); 56 percent left-side weight; Panhard bar at 11 inches left and 12 inches right, mounted to the right side of the chassis; and no front Ackermann with zero rear steer. On this car, we would have already moved the upper mounts for the rear shock/springs out as much as possible to reduce the rear spring angles.

On a track that has good moisture and is smooth, the car will be easy to control going through the turns and the speeds will be high. We keep the low CG (mount the weight low in the car) to reduce weight transfer to the right side to help maintain left-side weight in the turns.

HS: We could maintain the previous setup as the track begins to smooth out, but it will keep a considerable amount of grip. We could raise the rear ride height an inch or so to be ready if the track begins to dry out some and affect bite off the corner. Keep the all-terrain tires and lower the pressures a bit.

7/15As the track goes more toward the slick condition, we begin to soften the right-side springs and make changes to the rear moment center heights.

LM: These conditions are the fastest you will encounter. The car should enter, go through the middle, and come off the corners going straight ahead and balanced in the overall setup. We will make some changes to the previous spring setup. A typical setup might be: LF = 350; RF = 375; LR = 175 (325 pounds swing arm), RR = 225 (400 swing arm); 56 percent left-side weight; Panhard bar at 11 inches left and 12 inches right; and no front Ackermann with zero rear steer.

We will need to change to tires that are harder, with less gap and more surface rubber. The air pressure must come down some, but do not run soft on air at this point. Zero rear steer is essential now because our dirt car will handle much like an asphalt car. The driver can drive straight into the corner, brake harder, and get back into the throttle sooner than at any other time of the day.

8/15We also move the Panhard bar chassis mount over to the left side at this point.

Under these conditions, teams using softer RR springs with Panhard bars high on the left side and low on the right side and with plenty of rear steer will find trouble. In short, they will be all over the track.

Once the track has hardened, it will quickly continue to change toward black and then to slick. The hardened track condition may only occur for one heat or so. We might want to anticipate the dry and slicker conditions coming now for lack of time later.

HS: As the track begins to dry out, the surface will start to become black and a little slick. We can now begin to soften the setup somewhat and change the RF spring to an 800-pound (rated) spring or remove the RF full spring rubber and replace it with a half rubber. The RR spring should change to a 200-pound (rated) spring or pull one rubber out. Raise the rear ride height another inch. This does two things: It lowers the rear moment center and raises the car's CG. The tires can be changed to more of a street performance tread with slightly lower pressures.

9/15Once the track has developed a black groove, we are fast on the way toward a slick track. The groove is usually narrow, and the area outside the groove becomes slick as the material dries out. The right-side springs are now softer than the left-side springs.

Evening up the front springs will help the car turn, and softening the RR spring, lowering the moment center, and raising the CG will provide better bite as the track dries out.

LM: We can now begin to change the spring rates at the RF, RR, and LR. The RF = 350; RR = 200; LR = 200 (375 pounds swing arm). We can move the Panhard bar over to the left side of the chassis with the heights at 11 inches left and 9 inches right. We can start moving weight over to the right side to provide a lower left-side weight of 52-54 percent. Now softer tires can be used with some siping. Slightly increased Ackermann and small amounts of rear steer (LR tire coming forward) will help the car turn through the middle.

The track has now blackened over and the edges of the groove are loosening up with the surface material turning to drier sand. Passing becomes difficult because of the one-groove nature of the track. The "slide job" works well here because as you go under a car coming into the corner, you pass and inevitably slide in front of the passed car. Once your car hits the black groove, it will grip and you can accelerate off from the passed car.

10/15Now the track has gone to an overall slick condition that requires more imagination.

HS: When the track turns black, the surface has become hard with very little loose material. The moisture is evaporating off the first inch of material, and there will be less available grip without the surface becoming overly slick. We will still need to drive the car straight and try not to soften the right-side springs too much yet. We won't change the setup at this point in time, but the driver will need to think more about throttle control coming off the corners to prevent wheel spin.

LM: We will want to soften the setup a little more while the track is transitioning through this phase, and we need to reverse-split the front springs. We could continue to move weight up and to the right in the car to raise the CG about 1 inch and further decrease the left-side weight percentage. The LF spring = 375; RF = 350; LR spring = 185; and RR spring = 175.

11/15We can utilize the entire width of the track for passing or to find the fastest groove. Bite off the corners will be at a premium.

The Panhard bar, mounted on the left side of the chassis, should be at heights of 12 inches left and 8 inches right. Tire selection should include a little more siping to help dissipate heat, as the tires will begin to spin slightly coming off the corners. There will be less grip, but not to the point of going overly soft on the springs or introducing great amounts of rear steer to turn the car.

The track has now dried up and the black is mostly gone. The surface has not come apart just yet, and passing is a bit more manageable because the entire width of the track is uniform in grip. If the track changes banking with more angle on the high side, then passing can be done on the outside more easily than down low if the car is handling well.

12/15We have now reached the final stage in the track's transition. The surface has dried out completely, and fine particles of material (sand) cover the surface, reducing the traction to very low levels.

HS: We can now soften the right-side springs, or pull out all of the spring rubbers. It is time to soften the car, move weight over to the right, and lower the rear moment center more. Change the RF spring to a 750-pound spring and the RR to a 175-pound spring. Raise the rear ride height one more inch, move weight up and to the right in the car, and soften all of the shocks. Reduce the air pressure in the tires by 2 pounds all around.

LM: It is time to go a little more radical in the setup. We will soften the RR spring and the front springs. LF = 350; RF = 325; LR = 200 (350 pounds swing arm); RR = 175 (325 pounds swing arm). Raise the Panhard bar by 1 inch to 13 inches left and 9 inches right. Increase the rear steer by changing the angles of the left trailing arm. Decrease the air pressure all around for more footprint area, and groove the tires for the most side bite with more grooves and a limited number of siping cuts.

13/15We need to incorporate rear steer in greater amounts to improve entry performance as well as take advantage of the aero affect of running the car at an angle (top view) to the oncoming air.

Install softer rebound left-side shocks and softer compression right-side shocks. Raise the moveable weight up and to the right side as much as possible for a 50-50 percent side-to-side weight distribution. Increase the Ackermann to 1 degree in 15 degrees of steering. The LF tire will be doing less work with these changes but should still be touching the ground, and the Ackermann will help it provide turning power.

Now we have come all the way to the most difficult conditions we will see in dirt track racing. We must get radical now, but not to the point of jacking the LF corner of the car off the ground. We need to maintain the downforce our car produces for corner speed, while providing some kind of bite off the corners.

HS: With the track going to full dry and slick conditions, we need to get more radical in our setup. We will now soften the LF spring to a 750, the RF spring to a 650, the RR spring to a 150, and move all of the moveable weight to the right side and higher up. We will take more air out of the tires, but not enough to cause them to roll over. We should soften the shock rates all around so that the "shock" to the tires will be reduced as the car transitions going into the turns and accelerates off the corner. Smooth driving is important now as any complete loss of grip in the front or rear tires will cause a great increase in the lap times. Soft driving seems slow, but the alternative is always slower.

14/15

LM: We definitely need to soften up the setup and introduce more rear steer to be able to negotiate the turns without causing the LF tire to be lifted completely off the track surface. When the LF is hiked up considerably, we lose any front downforce that we might have enjoyed, as a great deal of air comes under the raised LF spoiler and we lose most of our front grip. There is only one way to stop a severe pushy or loose car, and that is to lift off the throttle. We can observe cars that three-wheel off the turns and hear the throttle being cycled on and off.

15/15We can introduce a lot of rear steer, if needed, by positioning the left-side links in the four-bar car all the way to the top of the adjusting holes. We usually do not want the RR to steer because that is the driven wheel off the corners and needs to remain stable in its position fore and aft.

A more sideways driving style is sometimes helpful for mostly aerodynamic reasons. Putting the large, flat side of a Late Model into the wind has a pronounced effect on slowing the car and creating lateral lift. Lower pressure on the left side will provide a lateral force that counters the centrifugal force that is trying to take the car up to the wall. That is probably why rear steer (left tire forward) is sometimes so helpful. As the car rotates, it develops an aero force that acts like added traction. This effect can be taken to an extreme, and we never want to three-wheel the car in order to create the effect.

In our journey through a full day of dirt track racing at "Anywhere Speedway," we have purposely left out a number of setup variables such as fifth and sixth coil adjustments, gear changes, birdcage indexing, rear spring placement (front or rear of the axle tube), the decision to clamp or not to clamp, and others. Opinions will vary on these topics. Our intent was to provide some thoughts on making changes to the basic setup that might improve the car and make each phase better.

The key, again, is to be sure to make changes based on your track conditions related to your type of car. To run a single setup throughout the day is not going to get the job done in this day and age. Racers of all classes are getting smarter by the year, and getting to the front is a matter of staying on top of the most current technology. It's what racing is all about and it's all fun. If we had nothing to learn, we may as well go golfing-not.